The present invention relates generally to protective circuitry, such as overload circuits. More particularly, the invention relates to a processor-based overload relay that is self-powered by virtue of power management components, permitting power to be stored for its operation, while performing measurements of voltage and current.
Overload relays and similar circuits are used in a wide range of settings. For example, applications involving powering electric motors, a motor starter or motor controller is typically coupled to a motor to supply single or three-phase power. The motor drive, in many applications, may synthesize an output waveform to vary the frequency of the drive power so as to permit the motor to be driven at various speeds. The waveform may also be manipulated to control torque, and so forth. Motor controllers, however, do not typically provide for interrupting power to the motor in case of need. Depending upon the circuit configuration, other protective devices typically serve this purpose. Such devices may include fuses that are often positioned upstream of all other circuitry and downstream of a power supply, such as the electric power grid. The fuses may be supplemented by magnetic and thermal overload circuitry. Magnetic overload circuitry typically trips to open the power circuit in response to rapid changes in current. Other overload circuitry may operate more slowly, and may model motor windings or other wiring, to permit opening of the circuitry should longer-term rises in temperature be detected or estimated.
In the area of overload relays, a number of different configurations have been developed and are presently in use. Such relays can vary from quite simple electro-mechanical devices to more sophisticated circuitry that incorporates application specific integrated circuits (ASICs), or processors, typically microprocessors. Such ASICs and processors offer a significant benefit in being capable of analyzing current and voltage data and judiciously opening or closing power circuits based upon the analysis. Where possible, sophisticated yet high production (and thus cost effective) processors, including microprocessors and field programmable gate arrays may be used for such purposes.
One challenge in the use of such circuitry, however, is the ability to provide sufficient power for its operation. Specifically, smaller sizes of overload relays may not be able to provide sufficient power for operation of microprocessor-based control circuitry. In many cases, it is advantageous to power the circuitry from power that is extracted or scavenged from the sensing devices, such as current transformers. However, where power levels required for the processing exceed the available power budget, more costly and less flexible ASICs and other circuitry may be required.
There is a need, therefore, for improved circuit designs that may permit the use of more sophisticated processing capabilities that are powered by current transformers and similar power scavenging devices. There is a particular need for such circuitry that may permit microprocessors and similar circuits to be used in small electro-mechanical devices, such as overload relays, that have a reduced power budget.